Industrial gases such as oxygen, nitrogen, hydrogen, carbon monoxide and hydrocarbons as well as other industrial gases are the subJect of intense competition to arrive at the most efficiently produced and low cost product as is possible to provide. Many industrial gases have traditionally been produced in large scale plants using cryogenic techniques. The cryogenic supply of industrial gases can be expensive when transportation of such gases from a cryogenic process facility to an end use customer is considered. With the change in users of industrial gases from large basic industries, such as the steel industry with their large consumption needs, to industries which require smaller quantities of industrial gases, cryogenic facilities see greater competition for reduced volumes of industrial gases from alternate industrial gas processing facilities, such as membrane and sorptive separation systems. Sorptive separation systems typically constitute the use of pressure swing to regenerate a plurality of parallel sorptive beds. Unless a large plurality of parallel beds are utilized, pressure swing sorptive systems typically render discontinuous product production. Additionally, customer demand or utilization of industrial gases from pressure swing sorptive systems is typically variable. Problems can arise in maintaining a constant product gas flow from the sorptive system when its discontinuous production is compounded by the variable demand of the ultimate industrial gas user.
Changes in product flow from a pressure swing sorptive system alter the purity produced from the system because of the change in the rate of gas permeation through the sorptive beds. Therefore, the industry has sought ways to achieve constant product purity or at least constant product flow during conditions of discontinuous production and/or variable product withdrawal.
U.S. Pat. No. 4,323,370 discloses an adsorptive process wherein the length of time of operation of the adsorption phase and the rate of flow of a gaseous feed mixture through the adsorber is adjusted in response to fluctuating demand for the product gas.
U.S. Pat. No. 4,576,614 discloses a pressure swing adsorptive separation system utilizing two parallel beds and a product reservoir. Product pressure or flow is standardized by discontinuing the initiation of a new adsorptive step until prescribed values of product gas in the reservoir are reached.
U.S. Pat. No. 4,725,293 discloses a pressure swing adsorption system having two parallel beds and a product reservoir wherein conditions are sensed in the product reservoir and product line and through computer controls the extent of feed gas to the adsorptive system is modified and controlled to result in maintenance of a preselected range of impurity in the product and control of product flow at a relatively high predetermined rate.
Additional patents of interest in this area include U.S. Pat. No. 3,230,688; U.S. Pat. No. 3,257,772; U.S. Pat. No. 3,282,027; U.S. Pat. No. 3,703,068; U.S. Pat. No. 3,778,967; U.S. Pat. No. 4,140,495; U.S. Pat. No. 4,197,095; U.S. Pat. No. 4,299,595; U.S. Pat. No. 4,349,357; U.S. Pat. No. 4,376,640; U.S. Pat. No. 4,494,966; U.S. Pat. No. 4,504,286; U.S. Pat. No. 4,539,019; U.S. Pat. No. 4,552,571; U.S. Pat. No. 4,561,287; U.S. Pat. No. 4,561,865; U.S. Pat. No. 4,576,614; and U.S. Pat. No. 4,643,743.
The prior art has attempted various techniques for maintaining product purity and product flow in an adsorptive separation system. However, these systems are either inadequate, complex or capital intensive and do not meet the needs of a low cost, simple pressure swing adsorptive system particularly wherein a minimum of parallel beds are utilized. The present invention overcomes the drawbacks in a simple, low cost sorptive separation system using a minimum of beds, wherein discontinuous production and variable product demand are experienced, to result in constant product flow at a prescribed minimal range of impurity fluctuation. These desirable attributes are achieved in the present invention which is set forth in greater detail below.